The user is allowed to enter 1000 ids in the array, but the default number of parameters that EclipseLink allows in a query using "in" is 256. In EclipseLink 2.1 there's a method called setBatchFetchSize() to tweak this value, but I'm stuck using 1.1.4.

At least I assume that's why I only get 256 objects in my reply, though the full 1000 ids are logged...